1. Field of the Invention
The present invention relates to a production apparatus and a production method for crankshafts. In particular, it relates to an improvement in a technique for forming of plural hollow hole portions to a crankshaft for weight reduction thereof.
2. Description of Related Art
A crankshaft of internal-combustion engine has a journal shaft portion. A crankpin portion parallel to the journal shaft portion is connected to the journal shaft portion by arm portions. A counterweight portion is formed to the arm portion. In the arm portion, the formed position of the counterweight portion with respect to the journal shaft portion is opposite to the connected position of the crankpin portion. In the crankshaft, in order to improve fuel consumption, a hollow hole portion may be formed to the crankpin portion for weight reduction thereof. Even when the hole portion is formed to the crankpin portion, the influence on the stiffness of the crankshaft is small, so that it is desirable to form the hole portion to the crankpin portion.
In formation of a hole portion to a crankpin portion, a forging apparatus may be used. The forging apparatus has a side forming punch which moves in a direction perpendicular to a movement direction of a press ram. A cam mechanism has been used as a driving source for the side forming punch of the forging apparatus as disclosed in Japanese Unexamined Patent Application Publication Nos. H1-104436 and 2003-343592. The cam mechanism has a mechanism which is simpler than that of servomotors and hydraulic apparatus, the cam mechanism is not provided outside a die set, and the cam mechanism allows the side forming punch to linearly follow the action of the press ram.
FIG. 7 is a conceptual diagram for explanation of action of a side forming punch 20 (hereinafter referred to as “punch 20”) by a cam mechanism 10. The cam mechanism 10 is equipped with a cam 11, a cam driver 12, and a cam holder 13. The cam driver 12 drives the cam 11. The cam 11 and the cam driver 12 are slidably supported by the cam holder 13. The punch 20 is provided at a side surface of the cam 11 which faces an inside of a die, and a side surface of the cam 11 which faces an outside of a die is an inclined surface. A lower surface of the cam driver 12 is an inclined surface. In an initial condition, the lower surface of the cam driver 12 is positioned so as to be spaced a predetermined distance from the inclined surface of the cam 11. In action of the cam 11, the lower surface of the cam driver 12 slides on the inclined surface of the cam 11. In the cam mechanism 10, when an upper plate 31 moves a predetermined distance downwardly to a lower plate 32 by a press ram (not shown in the Figure), the inclined surfaces of the cam 11 and the cam driver 12 contact each other. When the upper plate 31 moves further downwardly, the inclined surfaces of the cam 11 and the cam driver 12 slide on each other, and the punch 20 moves toward the inside of the die in a horizontal direction.
However, when a crankshaft is formed by the above forging apparatus, it is necessary that the punch 20 be inserted into the crankpin portion from an axial direction of the crankshaft since the crankshaft is disposed in the die such that the axial direction of the crankshaft should be perpendicular to a movement direction of the press ram. Due to this, when a hole portion is formed to each crankpin portion of the crankshaft having plural cylinders, punches collide with each other.
Specifically, in side forming which uses the cam mechanism 10, the cam 11 acts in accordance with the movement of the press ram, so that the inserted length of the punch 20 provided on the cam 11 is maximum when the press ram arrives at a bottom dead point. Next, when the press ram moves to a top dead point, the punch 20 is removed from the crankpin portion. Due to this, for example, as shown in FIG. 8, when a hole portion 44 is formed to each crankpin portion of crankshaft 40 which is used for four cylinders and has a full counterweight structure, punches 21 and 22 collide with each other, and punches 23 and 24 collide with each other (as shown in a portion surrounded by a dotted line in FIG. 8). As a result, during one stroke of the press ram from the top dead point to the bottom dead point, plural hole portions, which are positioned such that the punches for forming of them interfere with each other, cannot be formed. Reference numerals 41 and 42 denote a journal shaft portion and a crank arm portion.